Display devices such as liquid crystal display devices, light-emitting diode (LED) display devices, video display units (VDUs) and the like are common in many areas of work and everyday life in the industrialized countries of the world. It is not unusual for such devices to display information which is private and confidential, and intended only for the benefit of a user of the display device, or for a limited group of people who must work with the information displayed.
Such a display, for example, may be found in an automated teller machine (ATM) outside of a bank. Such a display may be a VDU in an office displaying, for example, personnel records, medical records, or other confidential information. In many cases, it is not possible or practical to restrict movement of people in the vicinity of the display who have no need to see the displayed information. In such cases, there will always be a possibility that the displayed information may be seen by unauthorized people who may use the information to the disadvantage of a person or persons that the information concerns.
There is a need for a relatively inexpensive filter which will restrict visibility of an image on a display device to a narrow range of viewing angles near normal incidence, while obscuring the image at angles outside of the range. Such a filter would limit the extent to which displayed information could be seen by a casual or unauthorized observer. Preferably, the filter, from a users point of view, should be effective in reducing reflection from the display device thus providing contrast enhancement for the displayed image.